1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for directional designature of seismic data collected with one or more streamers.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, improving the resolution of images of the structures under the seafloor is an ongoing process.
During a seismic gathering process, as shown in FIG. 1, a vessel 110 tows plural detectors 112. The plural detectors 112 are disposed along a cable 114. Cable 114 together with its corresponding detectors 112 are sometimes referred to, by those skilled in the art, as a streamer 116. The vessel 110 may tow plural streamers 116 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to the surface 118 of the ocean.
Still with reference to FIG. 1, the vessel 110 may tow a sound source 120 configured to generate an acoustic wave 122a (or another type of wave, e.g., electromagnetic). The acoustic wave 122a propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector R). The reflected acoustic wave 122b propagates upward and is detected by detector 112. For simplicity, FIG. 1 shows only two paths corresponding to the acoustic wave 122a. However, the acoustic wave emitted by the source 120 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 120.
In other words, parts of the spherical wave propagate directly, see seismic wave 123, to the detector 112. Parts of the reflected acoustic wave 122b (primary) are recorded by the various detectors 112 (the recorded signals are called traces) while parts of the reflected wave 122c pass the detectors 112 and arrive at the water surface 118. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), the reflected wave 122c is reflected back toward the detector 112 as shown by wave 122d in FIG. 1. Wave 122d is normally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 112, but with a reverse polarity and a time lag relative to the primary wave 122b. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content recorded by the detectors.
The recorded traces may be used to determine the subsurface (i.e., earth structure below surface 124) and to determine the position and presence of reflectors 126. However, the recorded traces include a combination of the desired earth reflectivity and the source signature, or far-field signature. It is desired to remove the far-field signature from the recorded seismic data, a process known as “designature.”
Far-field designature is a standard step in the marine processing sequence which converts the source far-field signature to a desired output. The conversion is made by convolving the data by the derived shaping filter. Usually a filter is derived to combine the operations of debubbling and zero-phasing. This approach leaves the source ghost notch in the spectrum of the data and produces a tight zero phase wavelet. The far-field signature is often derived using modeling software (e.g., Nucleus (PGS Seres AS) or Gundalf (Oakwood computing associates Ltd)).
In the quest for broader bandwidth data it is necessary to deghost the data on both the source and receiver sides in order to pursue the true subsurface reflectivity. For conventional data, there is a limited diversity of the receiver ghost notch frequencies which often prevents effective deghosting. For this reason, more sophisticated solutions have been developed which include over-under streamers, variable depth streamers, and utilizing streamers incorporating geophones as well as hydrophones.
On the source side it has also been necessary to move towards ghost removal. For conventional source data, this means shaping the far-field signature to a high bandwidth zero-phase pulse. More recently, broadband sources have become available and they use airguns at more than one depth to diversify the source ghost. Usually, designature is applied as a 1D filter even though the source response is not isotropic. To achieve the correct broadband results for all angles, it is necessary to apply full directional designature where the source signal at all take-off angles is corrected to the same zero phase wavelet.
Although designature is commonly 1 D, to properly compensate for the directivity of the source, directional designature is necessary. This improves resolution and properly preserves AVO. This may be achieved by making a plane wave decomposition of the data in the common receiver domain. This transformation produces a different trace for each source take-off angle which allows the application of angularly dependent filters. Such schemes have been applied in 2D in the tau-p domain and in the f-k domain. However, as the plane wave decomposition is a weighted sum of traces from different shots, this approach is only strictly valid if the directional signatures do not change from shot-to-shot. As a consequence, it is often assumed that the directional signatures remain constant throughout the whole survey.
An existing way of partially solving this problem is to apply 1D shot-by-shot designature in the time-offset domain, followed by a global directional designature in the receiver tau-p domain. However, this approach does not properly correct for shot-to-shot directional designature effects.
Thus, there is a need for a new method that overcomes the above noted deficiencies while at the same time, achieves full shot-by-shot directional designature on a shot-by-shot basis.